Reservoirs containing heavy oil (e.g., hydrocarbons having a viscosity above about 1500 cP at reservoir temperature and/or an asphaltene content above about 2% by weight) sometimes have compositional gradients. Where such reservoirs are thick (e.g., having a vertical extent exceeding 20 meters), the effect of the compositional gradients may be amplified. For example, the compositional gradients may cause changes in viscosity, temperature, asphaltene content, fluorescence intensity, and/or other parameters as a function of depth, perhaps changes having several orders of magnitude. Thus, downhole fluid analysis (DFA) utilizing optical spectroscopy may be performed. However, scattering caused by emulsified water, which can dominate the optical absorption, may complicate optical spectrometry with heavy oils. As a result, DFA answer products available for conventional oils may not be available for heavy oils.